1. Field of the Invention
The present invention relates to a DC/AC inverter substrate for controlling the lighting of a liquid crystal backlight, and more particularly, to a DC/AC inverter substrate having a voltage abnormality detector circuit which is capable of detecting a voltage abnormality.
2. Description of the Related Art
A liquid crystal display device requires backlight as a light source because the device is non-luminous. As the light source of the backlight, there is used a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED). In order to light those light sources, the liquid crystal display device has a DC/AC inverter substrate on which a lighting circuit is mounted. The DC/AC inverter substrate includes a DC/AC inverter circuit that generates a high AC voltage from a DC voltage and a lighting circuit as well as a protection circuit. The protection circuit detects a current that flows in the light source, and stops the output of a high AC voltage when a current that flows in the light source becomes abnormally high.
The DC/AC inverter substrate (hereinafter also abbreviated as “INV substrate”) is shown in FIG. 1. FIG. 1 shows a configuration example including four transformers each of which operates a lamp. The DC/AC inverter substrate is equipped with an input connector 11 that receives a DC voltage, output connectors 12 that output an AC voltage, transformers 13 that conduct DC/AC conversion, an overcurrent fuse 14 that protects the circuits against overcurrent, temperature fuses 15 that protect against overheat at the time of the abnormal output of the transformer, and tube current detector circuits 16 that feed back currents (hereinafter referred to as “tube current”) from lamps which are connected to the output connectors 12, and stop the output when the tube current becomes equal to or higher than a given value.
For example, in the case where the output connectors 12 are opened due to the disconnection of the connectors, the tube current detector circuits 16 detect the current, and stop an inverter output circuit as abnormality. Also, when overheat occurs due to the abnormal output of the transformer, the temperature fuse 15 is burned out to stop the inverter output. When the DC input is overcurrent, the overcurrent fuse 14 is burned out to stop the inverter output. In this way, the overcurrent and the overheat of the INV substrate are detected to stop the inverter output, thereby preventing the INV substrate and the liquid crystal display device from being damaged.
For example, JP 2005-183099 A (hereinafter referred to as “Patent Document 1”) discloses an electric discharge lamp lighting circuit having a protection circuit. In the case where electric discharge occurs in the secondary wiring of a high voltage transformer of the electric discharge lamp lighting circuit, the operation of the electric discharge lamp lighting circuit stops upon detection of the electric discharge. Patent Document 1 also proposes an inexpensive electric discharge lighting circuit which is configurable for a large-sized liquid crystal display device.
An electric discharge lamp lighting circuit 51 disclosed in Patent Document 1 is shown in FIG. 2. The electric discharge lamp lighting circuit 51 includes a control circuit 53 that is connected to an input voltage line 52, and a transformer driver circuit 54 that drives a transformer 55. Also, one terminal of an electric discharge lamp 56 is connected to one terminal of a secondary side of the transformer 55, and another terminal of the electric discharge lamp 56 is connected to a current/voltage converter circuit 57 for converting a lamp current into a voltage, and a lamp current control pattern 58. An electric discharge detection pattern 61 is disposed in parallel to and in proximity to the lamp current control pattern 58 at a GND side of the secondary side of the transformer 55. An overcurrent detection resistor 59 is connected between the GND side of the secondary side of the transformer 55 and the electric discharge detection pattern 61. Both ends of the resistor 59 are connected to the respective anodes of diodes 60 and 62, and the respective cathodes of the diodes 60 and 62 are connected to each other, and also connected to the control circuit 53 through an integrator circuit 63. The integrator circuit 63 can be made up of a resistor 64 and a capacitor 65.
With the above configuration, corona discharge or arc discharge can be detected by a voltage that is induced in the electric discharge detection pattern 61 due to the high frequency components and the noise components of the lamp current that flows in the lamp current control pattern 58. When the induced voltage is detected, the operation of the electric discharge lamp lighting circuit stops to protect the electric discharge lamp lighting circuit. Because the electric discharge detection pattern is disposed at the GND side of the secondary side of the transformer, it is unnecessary to provide an independent electric discharge detection pattern, thereby making it possible to simplify the electric discharge lamp lighting circuit. Also, the electric discharge detection pattern is formed not linearly but zigzag, thereby enabling the provision of the inductance of an arbitrary pattern. As a result, the electric discharge can be more efficiently detected.
Further, JP 2000-83319 A (hereinafter referred to as “Patent Document 2”) discloses the protection circuit of a power supply device, and a detecting device in the conventional art. Patent Document 2 discloses the power supply device having heating prevention protecting means which is made up of a temperature sensor. JP 2001-14093 A discloses a contact detecting device using a planar interconnection and a connector for an electric connection. JP 2001-286161 A discloses a power supply device that monitors the output of a piezoelectric transformer.
JP 2002-22777 A discloses an optical system sensor device that measures the electric quantity of transmission and distribution electric wires such as a distribution electric wire or a transmission electric wire by the aid of the property that a current and a voltage act on light. Optical sensor heads are fixed to the respective transmission and distribution electric wires, read a change in the voltage or current of the transmission and distribution electric wires, and transmit the read change as electric signals.
JP 2003-28823 A discloses a signal processing device that alternately applies a higher voltage and a lower voltage to two sensors that are indicative of high sensitivity to a specific gas according to an applied voltage. JP 2006-105955 A discloses an energization detecting device in which a pair of sensors are arranged in proximity to the outside of a power supply cable, and detects the presence or absence of the energization according to a voltage between both of the electrodes. JP 09-264792 A discloses a noncontact temperature sensor that corrects a temperature difference between a front surface side and a rear surface side of the substrate by detecting the front surface and the rear surface thereof.
As described above, the liquid crystal display device includes the INV substrate on which the DC/AC inverter circuit having the high voltage transformer is mounted in order to light the backlight. In the INV substrate, there is a risk that a failure of connection between the secondary side terminal of the high voltage transformer and the wiring, a disconnection of the wiring, or a failure of connection with the lamp occurs. When an interval of the failure portions is narrow, there arises such a problem that the corona discharge or the arc discharge is generated in the failure portion to damage the INV substrate or the liquid crystal display device. Under the circumstances, the protection circuit is disposed on the INV substrate.
However, for example, the protection circuit disclosed in Patent Document 1 is insufficient, and suffers from the following problems. In Patent Document 1, in order to detect the corona discharge and the arc discharge, the electric discharge detection pattern and the lamp current control pattern are disposed in parallel to and in proximity to each other. The electric discharge detection pattern is disposed at the secondary side output of the transformer, and the lamp current control pattern is disposed in parallel to and in proximity to the electric discharge detection pattern of the output of the electric discharge lamp.
As a first drawback, the corona discharge or the arc discharge in the secondary side wiring of the transformer can be detected by the electric discharge detection pattern and the lamp current control pattern. However, there is no disclosure of the technique for detecting the corona discharge or the arc discharge at the low voltage side of the lamp, and the electric discharge at the low voltage side of the lamp cannot be detected. Also, there is no specific disclosure of the details of the pattern, for example, the pattern layout position relative to the transformer or the lamp connection point, or the pattern sizes. For that reason, when the respective patterns are positioned apart from a portion where the corona discharge or the arc discharge is generated, the abnormal voltage of electric discharge attenuates down to approximately the same voltage level as the normal voltage, and cannot be detected.
As a second drawback, there is the necessity that a position at which the electric discharge detection pattern and the lamp current control pattern are arranged on a printed circuit board is restricted. For example, there is a case in which the respective patterns cannot be disposed in parallel to or in proximity to each other depending on the arrangement of parts that are mounted on the printed circuit board. On the contrary, when an attempt is made to dispose the respective patterns in parallel to or in proximity to each other, the sizes of the printed circuit board need to be given larger sizes taking the mounted parts into consideration, thereby making it difficult to reduce the sizes of the printed circuit board. In other words, this system is disadvantageous in downsizing the printed circuit board.
As a third drawback, the abnormal voltage gets into the electric discharge detection pattern, and damages the pattern. As a result, the abnormal voltage cannot be detected. The electric discharge detection pattern is electrically connected directly to the secondary side output of the transformer. For that reason, when the corona discharge or the arc discharge occurs, the abnormal voltage gets into the electric discharge detection pattern. As a result, the electric discharge detection pattern is damaged, and the abnormal voltage cannot be detected. Similarly, because the lamp current control pattern is also electrically connected to the output of the electric discharge lamp, when the corona discharge or the arc discharge occurs, the abnormal voltage gets into the electric discharge detection pattern. For that reason, the pattern is damaged, and the abnormal voltage cannot be detected.
As described above, in the protection circuit disclosed in the prior art document, it is insufficient to detect the abnormal voltage which is attributable to the abnormal discharge, and a more effective protection circuit is expected. In view of the above circumstances, an object of the present invention is to provide a DC/AC inverter substrate having a voltage abnormality detector circuit which is capable of particularly detecting the voltage abnormality which is attributable to the abnormal discharge.